30 THE GENERAL CHARACTERS OF CELLS, BY S. STRICKER. 



completely closed, whilst the other exhibits a roundish, sharply 

 defined opening. 



At a much earlier date the epithelial cells of the villi had 

 already been described by Briicke* as destitute of a membrane 

 at their free extremity. It is still a subject of dispute whether 

 this is a characteristic of all the epithelial cells, or occurs only 

 in certain cup-like organs scattered amongst the ordinary 

 cells of the epithelium. 



The cell membrane may appear perfectly homogeneous, or 

 it may possess pores (Leuckart).-f- 



The investing membrane may further present, according to 

 F. E. Schulze, a want of homogeneity in consequence of becom- 

 ing thickened by a secondary deposit. The basal part of the 

 membrane of the epithelial cells of the villi, which is turned 

 towards the lumen of the intestinal tube, must be regarded as 

 a structure of this kind. 



It is also here a disputed question whether this basal border 

 is perforated by pores (Funke, Kolliker), or is composed of 

 rods, giving it a striated appearance (Brettauer, Steinach). 



THE NUCLEUS OF THE CELL. Since R. Brown discovered 

 the existence of a nucleus in vegetable cells, no remarkable 

 advance has been made in our knowledge of this structure. 

 Both Schleiden and Schwann held that the development of 

 cells proceeded from the cell nucleus, and before as well as 

 after their time the nucleus has always been regarded as 

 a structure playing an important part in the propagation 

 of cells. The objections have been already stated that were 

 urged by Briicke against the view that the nucleus must be 

 admitted as an indispensable attribute in our idea of a typical 

 cell. We, in fact, know nothing with certainty respecting its 

 physiological significance, nor regarding its physical peculiari- 

 ties. It is, indeed, well known that, as a general rule, when 

 a nucleated cell divides, the division first proceeds from the 

 nucleus, which elongates, becomes hour-glass shaped, and ulti- 

 mately constricted into two segments. Briicke adduces this 



* Denkschriften d. Wien. Akadem., Band vi. 

 t See also F. E. Schultze, loc. cit. 



